1. Field of the Invention
The present invention relates to a development device that develops an image with a developer, and an image forming apparatus including the development device, for example, a copying machine or a printer.
2. Description of the Related Art
A two-component development system that uses a mixture of non-magnetic toner and magnetic carrier as a developer has been widely employed in conventional electrophotographic image forming apparatuses, in particular, image forming apparatuses that form color images. The two-component development system offers advantages in image quality stability and apparatus durability in comparison with other existing development systems.
However, if a large number of sheets are printed with an extremely small image area, the amount of consumption of toner is too reduced, and toner remains in the development device for a long time while being agitated, which leads to a phenomenon that the toner deteriorates. In general, toner is externally added with another kind of functional particles (external additive) to reduce adhesion or apply charges. For example, consider that polyester resin-made base toner is externally added with an external additive. In this case, however, functional particles may come off due to rubbing of toner caused by long-time agitation or the original functions imparted to the base toner may not be exerted. The deterioration of toner leads to unevenness of an image surface or causes a problem of fogging.
As discussed in, for example, Japanese Patent Application Laid-Open No. 08-314253, one countermeasure against the above problem is discharge control. This control is to consume a predetermined amount of toner after image formation if toner is not consumed for a predetermined period. Further, at the time of discharging toner, a toner discharge amount is controlled by forming various patterns of latent images. As a result, toner that has deteriorated due to agitation is discharged only in a predetermined amount to prevent the toner from remaining in the development device for a long time and deteriorating due to agitation.
Further, according to a technique discussed in Japanese Patent Application Laid-Open No. 2001-343795, in order to forcedly discharge low-tribo toner remaining in a development device for a long time, a toner patch is formed if a drum potential Vs satisfies the following relationship: (blank portion potential)−(direct current (DC) component of development bias)≦Vs−(DC component of development bias)≦0. If a result of detecting the patch indicates that a toner discharge mode should be set, the drum potential Vs that satisfies the above relationship is applied onto the entire surface when no image is formed, and low-tribo toner is discharged.
However, in the case of using the above conventional techniques, operations of the apparatus should be suspended for toner discharge control after one image formation process or between image formation processes. In other words, downtime is necessary. To that end, Japanese Patent Application Laid-Open No. 2006-293240 discusses a technique of discharging deteriorated toner from a development device to a non-image formation region positioned outside the sheet width upon passing a small-sized recording material (sheet) to reduce downtime.
If the above structure discussed in Japanese Patent Application Laid-Open No. 2006-293240 is employed, the problem of downtime is solved, but the following problem remains to be solved. To begin with, particle size distribution of general toner used for an electrophotographic process is described. In general, an average particle diameter of toner is about 5 μm to 10 μm. However, even such toner having an average particle diameter of 5 μm shows particle size distribution with a certain level of variation.
The inventor of the present invention made extensive studies and found that, among toner particles that were rubbed for the same period, toner particles having a small particle diameter cause the above surface unevenness or fogging. In addition, the inventor found that such toner having a small particle diameter can be effectively discharged by reducing a potential difference Vback between anon-image formation region and a development sleeve, and a discharge efficiency varies depending on the potential difference Vback.
In addition, in the case of discharging toner to the non-image portion positioned along a recording material width direction so as to reduce downtime, as discussed in Japanese Patent Application Laid-Open No. 2006-293240, a toner discharge area is limited compared with the case of discharging toner after image formation or when no image is formed as in the conventional technique. As a result, if conventional discharge control is performed, toner having an average particle diameter in the development device is used for development. This may hinder efficient discharge and makes it difficult to reduce downtime as well as to reduce fogging.